This invention relates to HF alkylation. Further it relates to the alkylation of an isoparaffin with an olefin. In one aspect, this invention specifically relates to an improved HF alkylation processing scheme in which an isobutane rich recycle stream to an alkylation zone is prepared substantially free of undissolved HF acid.
It is well known in the prior art that catalytic alkylation using hydrofluoric acid or sulfuric acid as the catalyst has become an important chemical tool for preparing alkylated hydrocarbons and derivatives thereof. The commercial and industrial demand for these products is exemplified with the demand for isoparaffin hydrocarbons of gasoline boiling range and with the demand for alkyl-substituted aromatics suitable for conversion to surfactants, e.g., detergents, wetting agents, etc. The prior art process of alkylation generally is effected by contacting an isoparaffin hydrocarbon feedstock with an olefin hydrocarbon in the presence of a catalyst such as hydrofluoric acid in a suitable reaction vessel for conducting chemical reactions.
In practice, there have been numerous process schemes advanced by the prior art for accomplishing the alkylation reaction, but it is extremely difficult to achieve a process scheme which embodies all of the desirable features of a completely optimum reaction. Optimizing the alkylation reaction is complicated by the fact that the alkylation reaction if not carried out properly has many side reactions, such as polymerization, which destroys the effectiveness of the reaction and inhibits the production of commercial quantities of desired alkylate. Additionally, the reaction, in order to be carried out commercially, requires a tremendous amount of auxiliary equipment for the recovery of the alkylate product, for the regeneration and reuse of the excess catalyst, and for the recovery and reuse of the excess reactants which have passed through the reaction zone.
The catalytic alkylation process to which the present invention is applicable consists of a process in which a mixture of hydrocarbons containing isoparaffins such as isobutane, isopentane, and the like, and olefins such as propylene, butenes, isobutanes, and the like, are mixed intimately in the presence of a strong acid catalyst, such as hydrofluoric acid or sulfuric acid, at generally room temperatures or lower for sufficient time to complete the reaction. The effluent from the reaction zone contains saturated isoparaffin hydrocarbons of higher molecular weight or boiling point than the isoparaffin in the original mixture. For convenience, these higher molecular weight isoparaffin hydrocarbons which comprise the reaction product from the alkylation zone are called "alkylate." Isobutane has been used almost exclusively as the reactant isoparaffin because of its reactivity and availability to produce high quality alkylate product. In similar manner, among the olefins, butenes and propylenes have been used satisfactorily. In some cases, it is desirable to use solely propylene or butene as the olefin reactant.
As is typical in most commercial chemical plants, the reaction between the isoparaffin hydrocarbon and the olefin hydrocarbon is performed with an excess of reactant isoparaffin in the reaction zone. Accordingly, there is a large excess of the reactant isoparaffin hydrocarbon remaining in the effluent from the reaction zone. In the prior art, the effluent from the reaction zone is settled to form an HF phase and a hydrocarbon phase.
Because of the large amount of unreacted isoparaffin hydrocarbon which remains in the hydrocarbon phase of the reaction zone effluent, a portion of this material is recycled to the reaction zone. The hydrocarbon phase recycled contains alkylate and unreacted isoparaffins. The effect of recycling this material is to decrease the load on product fractionation facilities conventionally required to separate unreacted isoparaffin from alkylate. The idea of recycle of alkylate containing isoparaffin is not new. One embodiment of an alkylation process practicing recycle of alkylate-containing hydrocarbon is shown in U.S. Pat. No. 3,867,473 (Class 260/683.45), the teachings of which are incorporated by reference herein. Besides the portion of the hydrocarbon phase which is recycled to the reaction zone, at least a portion of the hydrocarbon phase is subsequently separated by fractionation into several product and recycle streams. This fractionation normally includes an isobutane stripper, from which several streams are withdrawn, including an overhead product stream, a recycle isobutane stream, a n-butane product stream, and an alkylate product stream. The recycle isobutane stream is passed from the isobutane stripper to the reaction zone to provide the excess isobutane required to produce alkylate of high quality.
It has been found that the quality of the isobutane recycle stream will affect the quality of alkylate produced. While prior art has shown that HF, including dissolved HF, in the isobutane recycle stream is harmful to alkylate quality, it has now been found that only undissolved HF is harmful to alkylate quality.